Free particle impact machining process and apparatus employing the same

ABSTRACT

An enclosed chamber carries a high speed rotating abrasive throwing wheel which discharges, by centrifugal force, abrasive particles through radially extending channels against a circumferential array of workpieces requiring machining, and positioned in the path of the high speed abrasive particles. The chamber is subjected to vacuum pressure or a low density atmosphere, with little aerodynamic loss due to windage effects on the high speed rotating wheel, low drag on the abrasive particles as they move freely from the channels to the work piece and no cushioning due to the entrapment of air between the particles and the workpiece.

United States Patent Putnam [72] Inventor: David T. Putnam, Raleigh, NC.

[73] Assignee: International Brmines Machines Corporation, Annonk, NY.

[22] Filed: May 28,1970

[21] Appl.N0.: 41,416

2,483,956 10/1949 Workman 2,532,136 11/1950 Zahn 3,423,888 1/1969 Brown ..51/319 51 July 11, 1972 2,261,988 11/1941 Gaebel..... ..,..5l/9 UX 2,996,846 8/1961 beliaert.... ...51/320 X 2,507,166 5/1950 Lehman ..51/9 3,205,620 9/1965 Woodworth 4.5 1/321 X Primary l-lxaminerHarold D. Whitehead Arrornq-Hanifin & Jancin and Carl W. Laumann, Jrv

[ ABSTRACT An enclosed chamber carries a high speed rotating abrasive throwing wheel which discharges, by centrifugal force, abrasive particles through radially extending channels against a circumferential array of workpieces requiring machining, and positioned in the path of the high speed abrasive particles. The chamber is subjected to vacuum pressure or a low density atmosphere, with little aerodynamic loss due to windage effects on the high speed rotating wheel, low drag on the abrasive particles as they move freely from the channels to the work piece and no cushioning due to the entrapment of air between the particles and the workpiece.

l3Clairm,5DrawingI1gures PATENTEDJUL 1 1 m2 SHEET 18F 3 FIG. 2

6 WP SM U U 0 P H M F O W LC L M A w W6 00 Q R m F W L U G E R N W 0T E R U w U S BEARINGS IOO OIL

ATTORNEYS.

PATENTEDJUL 1 1 m2 3.675 373 sum 2 or 3 INVENTOR DAVID T PUTNAM ATTORNEYS FREE PARTICLE IMPACT MACHINING PROCESS AND APPARATUS EMPIJOYING THE SAME BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to controlled machining of glass, ceramic and metal workpieces, and more particularly, to an improved abrasive machining with extremely high abrasive particle speed.

2. Description of the Prior Art Abrasive blast machining of certain hard, brittle materials has been accomplished on a small scale. Further, conventional blasting equipment has been used for many years in the related processes of deburring and surface preparation, particularly of metals.

Attempts have been made to devise proceses which accelerate abrasive particles to such a velocity that the kinetic energy developed is of a sufficient magnitude to dislodge material from the work piece in a controlled manner. Particle acceleration, to some extent, has been achieved by injection of the particles into a high pressure air stream and increasing the velocity of the air to a maximum by passing it through a nozzle. Even with increased pressure and noules designed for supersonic flow, limitations are experienced in attempting to increase particle velocity to values materially affecting the abrasion characteristics of the moving particles. In order to increase the velocity of the air flow, extremely high air pressures are required involving, in turn, the necessity of pressure resistant components, greatly increasing the cost. Further, as the pressure is increased, the volume of free air required in such a system becomes quite large. Because of the initial cost and operating costs of a air compressor capable of fulfilling such requirements, this constitutes a serious limitation where a large volume of high pressure air is required. Further limitations exist with regards to the nozzles employed in abrasive blast machining. In present nozzle designs, air, after passing through the entrance of the nozzle, expands toward the outlet end with the resultant increase in velocity. However, since the air loses density as it expands, an increase in air velocity within a nozzle appears to have little efiect upon the velocity of abrasive particle when in the flow. For instance, it has been shown, for various nozzle sizes and abrasive particle sizes and shapes, that the average velocities obtained are from 200 to 400 feet per second, while, at the same time, the final air velocities at the nozzle exit are considerably higher. Further, wear is another problem associated with nozzles. Nozzles employed in existing blast equipment are formed from boron carbide, tungsten carbide, ceramic and iron, and, while the carbide nonles resist wear much better to a much higher degree than other materials, the life of the nozzle is relatively short and would be reduced even more if it were possible to increase the velocity of the abrasive particles.

Thus, the use of high pressure air and to supply abrasive particles as part of a flow steam through a nozzle for high speed machining of work pieces, is not feasible. To reach the necessary high abrasive particle velocity, the volume of abrasive particles and/or the compressed air required to machine the part is so great that it cannot be accomplished economically, tolerances and finishes are not controllable to the degree required, and the wear of process component is so great that the process is neither practical nor economical.

SUMMARY OF THE INVENTION The present invention is directed to a free particle impact machining process involving the action of abrasive particles which strike the surface to be machined at high velocity. The rinciple features of the present invention involve the use of a high speed rotating wheel to sling the particles by centrifugal force to thereby increase removal rates from the work pieces being machined, and the use ofa vacuum or light gas environment In decrease the aerodynamic losses including the effeclivc drag on the abrasive particles, cushioning due to the entrapmcnt of the air between the particles and the work, and tlrng on the rotating wheel.

The individual work pieces are supported in a circumferential array and spaced outwardly of the channels carried by the rotating wheel for conveying the abrasive material radially from a central feed path. The rotor or wheel, including the wheel channels, is coated with l/ 16-inch urethane as is the wall of the housing. Preferably, a vacuum pump coupled to the encloser draws the vacuum on the chamber and is further connected to the lubricating oil circulating system through a regulator, to cause oil to flow to the bearings with the bearing cavities being at a somewhat higher presure than the chamber in which the rotor is mounted. This prevents any abrasive from working under the seals and into the bearings. Helium or a similar light gas may subsequently fill the enclosure or replace the air to provide reduced density at atmospheric pressure. The work pieces may be supported on an annular plate and each work piece may be angularly shifted relative to the abrasive path from the end of the channel to the chamber wall. The individual workpieces may have their faces machined at right angles to the path of the abrasive particles or may be rotated independently or collectively at some angle thereto depending upon whether the workpiece is to be machined by impact or by particle cutting or by a combination thereof.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. I is a perspective view, partially in section, of a machine employing the free particle impact abrasion process of the present invention.

FIG. 2 is a schematic view of the vacuum system employed with the process and apparatus of the present invention.

FIG. 3 is a perspective view, similar to that of FIG. I partially in section, illustrating the drive mechanism for the abrasion throwing wheel and the vacuum pump for evacuating the enclosure about the same.

FIG. 4 is an exploded, perspective view of one half of a channel defining spoke of the abrasive throwing wheel employed in the machine of FIGS. 1 and 3.

FIG. 5 is a plot of the removal rates for glass with various abrasive speeds and grit sizes for the process of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT The process of the present invention involves the movement of free particles at high speed toward a work piece, for impact therewith to effect, depending upon the angle of impact, removal of material from the surface of the work piece. Small fatigue fractures are created upon impact at a angle such that the stock is gradually erroded away and shearing or cutting occurs when the particles contact the work piece at much smaller angles on the order of 15. The removal rate is further determined by the ductility of the material, the size of the abrasive grit, etc. Important to the process of the present invention is the passage of the high speed particles within a low density environment, by either creating a vacuum environment or one constituted by the low density gas such as helium or hydrogen. In the process of the present invention, the material removal rate varies with the square of particle speed for any given abrasive, and the decelerating effect of drag on abrasive particle varies directly with the air density and inversely with the particle diameter. The kinetic energy of each abrasive particle is given by:

l 1 4-1rDd" K E mV 3 )V.

where m is the mass of the particle and V is the particle velocity, D is the particle density and d is the particle diameter.

Assuming the same percentage of this energy is effective in removing material for any given impact angle and particle geometry, the material removal rate (cubic inches of stock per pound of abrasive) will vary with the square particle speed.

The retarding force due to drag on a particle R is given by:

gy per foot of travel.

The Percentage of n rgy lost per foot of travel is therefore:

percent loss 100 X pC and simplifies to percent loss: x

Thus, the present invention resides in the effective use of small particles by simply reducing the air density by means of a vacuum pump or the employment of a light gas such as helium at atmospheric pressure as the atmosphere surrounding the moving particles. of course, when using small particles there is a tendency of fine dust to float in the air which is slow in settling whereas large particles fall rapidly. Furthermore, the decrease in the density of the medium in which the abrasive particles travel results in an advantage due to a decrease in the cushioning effect caused by entrapment of air between the impacting particles and the work piece and decreased drag on the rotor itself which is rotating at high speed. Further, movement of the rotor sets up air current eddies (if not subject to vacuum) which diverts or substantially alters the particle path and visibly affects the machining process.

Turning to the drawings, the abrading machine of FIGS. 1 and 3 achieves controlled machining of a great number of work pieces in simultaneous fashion whose surfaces face the high speed moving abrasion particles. The work pieces are stationarily mounted in a circumferential array about a central rotating abrasive throwing wheel or rotor 10. A large drum or cylindrical housing 12 formed of sheet metal or the like is fixedly supported (by means not shown) and has adjustably coupled thereto, on the inner peripheral surface 14, an annular L shaped work piece support or ring 16 which supports a plurality of spaced work pieces 18. Circumferentially spaced pins 19 are inserted in corresponding holes 21 to set the height of work piece support ring 16. In this illustrated embodiment they are merely clipped to the edge of the horizontal rim portion 20 by clip members 22. In this fashion, the work pieces 18 which are illustrated as comprising thin rectangular pieces of glass or metal instead of facing the rotating wheel 10, at 90 to the path of the individual abrasive particles 26, may be pivoted to the right or lefl. By shifting, there occurs a change in the impact or cutting angle of the individual abrasive particles 26 emitted from the channels 24 and moving radially across the interior ofthe container 12.

The sheet metal drum 12 forms a sealed enclosure including a sheet metal segmented bottom 28 which may be welded to the inside wall of drum l2, and an overlying, preferably removable cover 30. Cover 30 is flanged at 32 to closely overly the upper, open end of drum 12. The bottom wall 28 is shown in FIG. 3 as being flat, for simplicity purposes, but in actuality consists of a plurality of inclined, joined sector sheets 34 defining circumferentially spaced valleys which tend to receive the abrasive particles 26 allowing them to accumulate in piles 36 adjacent the circumferentially spaced particle recovery pipes 38 which deliver the particle to a common sump (not shown) for return to storage container 40. Container 40 is positioned coaxially of drum l2 and above the drum to facilitate gravity delivery of the particles 26 to the center of the rotating particle throwing wheel 10. Preferably, the individual abrasive recovery pipes 38 are coupled to a common delivery pipe (not shown) and thus to the storage sump. The delivery pipe may include valve means for selectively shutting off communication of the sump to the interior of the drum 12 during the machining process, although vacuum is preferably applied to the complete system.

The drum 12 may be provided with one or more cruciform support plates 42 constituting individual arms 44 which extend inwardly from the drum periphery, the support plates 42 acting in conjunction with the bottom plate 38 to support the high speed abrasive throwing wheel 10. In this respect, the upper plate 42 carries a collar and bearing 46 which in turn supports an outer tubular shaft or member 48, the tubular shaft 48 being fixedly coupled to wheel 10, and extending downwardly and passing through a lower bearing asembly (not shown) associated with bottom plate 28. The bottom end of the tubular shafi 48 fixedly carries a pulley 50. A fixed drive motor 52 is vertically oriented and carries a second pulley $4 on the end of the motor drive shaft 56 with a conventional belt 58 coupling the pulleys. Thus, energization of the electrical drive motor 52 results in high speed rotation of the abrasive throwing wheel 10. The abrasive material carried within container 40 and in particular within the enlarged portion 42 thereof falls by gravity through a small supply tube 60 which is stationary. A portion of tube 60 extends directly within the rotating hollow shaft 48. Tubular shaft 48 is provided with a number of circurnferentially spaced openings (not shown) which are aligned with the inner end of the distribution channels 24 forming portions of the abrasive slinging wheel 10.

In this respect, the wheel 10 is defined by a central hub portion 62 which rigidly couples the wheel 10 to the rotating tubular member 48. The channels 24 are formed by the hollow spokes 64 which radiate outwardly from hub 62, the individual spokes 64 being joined at their outer ends by upper and lower annular rims 68 and 69 respectively. The spokes increase in width from the hub 62 outwardly toward rims 68 and thus form divergent channels of rectangular configuration including side walls 70, and top and bottom walls 72. ln order to coat the interior walls of channels, each spoke 64 is preferably formed of two identical halves, a base 76, FIG. 4, and an overlying cover which are coupled together in any suitable manner as by belts or the like passing through aligned holes or openings 80.

In the manufacture of wheel 10, it is preferred that the wheel be made in segmented form that is, the upper rim 68 including radial spoke cover 78 be integrally formed and act as covers for a lower spoke base 76. In this manner, by reference to FIG. 4, it is seen that a solid male mold member 78 may be readily inserted within the channel cavity of lower spoke base 76, and suspended by multiple spacers 73 which are grooved at 75 so as to fit over edges 77 of the U-shaped lower spoke base 76. With the mold thus positioned, liquid urethane is forced into the cavity between mold 78 and the channel wall surfaces and the assembly is cured at 150F. After removing the mold by lifting the spacers, as shown, the edges of the urethane layer 82 are trimmed and the cover section 78 for each channel is rigidly coupled thereto in overlying fashion through the use of bolts (not shown) which pass through rim 69 and are received by holes on each side of the channel base member 76. In addition, it may be necessary to apply an external coat of abrasive resistant material such as urethane to the wheel while, the interior wall of the housing carries a thin sheet of thermoplastic material such as urethane on the order of one-sixteenth of an inch in thickness. Further, supporting ring 16 may be similarly coated. To reduce the power requirements to rotate the wheel at the desired velocities, the wheel may be constructed of an aluminum alloy or other light weight metal alloy which is capable of withstanding the high rotary speeds.

An important aspect of the present invention resides in the provision of a low density atmosphere within the container or drum l2 and in particular in the path of the moving particles 26 as they emerge from the outer end of the individual channels 24 and pass radially at high velocity toward the circumferentially spaced work pieces 18. Preferably, a motor driven vacuum pump 84, FIG. 3, has its intake pipe 86 passing through the wall of the drum or housing 12 with the end coupled to a suitable air filter 88 internally of the same. This is schematically shown in FIG. 2 wherein the vacuum pump 84 is coupled by pipe or line 86 directly to the filter 88 within the housing 12. In addition, a second conduit 90 is coupled via regulator 92 to a lubricating oil return sump 94. Since the abrasive distributing wheel preferably rotates at high speeds in a vacuum atmosphere within the housing or drum 12, it is necessary to maintain good lubrication to the bearings 46. In this respect, the present invention further involves the feeding of oil from oil supply 96 to the various bearings 46 by an oil supply line 100, the oil in turn passing from the bearings to the oil return or sump 94 via line 102. The regulator 92 provides only a slight vacuum in the oil return reservoir causing the oil to move under low vacuum pressure from the oil supply through the bearings 46 to the oil return sump. The bearing cavities are the somewhat higher pressure than the chamber defined by housing drum l2. Thus, any abrasive which might naturally tend to work under the seals associated with the r0- tary bearings of the wheel, is forced out from under the seal. Preferably, the bearing seals and vacuum seals (not shown) associated with the hollow rotor shafi 48 are of fluorocarbonous material so that the seal friction is minimized.

Due to high speed, the heat developed by air drag on the rotor, tends to burn out the bearings. it is necessary to reduce the density of the atmosphere through which the rotor passes. A relatively high vacuum pressure less than 1 psi is employed. Alternatively, the replacement of the external atmosphere with a light gas such as helium, whose density is such that at atmospheric pressure the effect would be the same. Due to the molecular weight of helium, even at l4.7 psia, the heat due to air friction is well below that tending to burn out the rotor bearings.

In particular, precision ball bearings, such as those used in the aircraft industry, should be employed. Further, controlled application of lubricant to the bearings is required, necessitating recirculation of the oil under a vacuum pressure for recirculation less than that surrounding the rotor. The wheel preferably is formed of matched halves coupled together by bolts. Dynamic balancing of the wheel is required due to the capability of causing the particles to be accelerated to speeds up to 1,000 feet per second.

For the rotor employed in the illustrated embodiment, the heat due to windage on the rotor and the windage itself involved a horsepower loss at 1,000 feet per second or about 500 horsepower.

The rotor is designed to operate at up to 9,000 rpm with power being supplied from motor 52 through pulleys 50 and 54; the electrical motor 52 constituting in the illustrated embodiment, a S-horsepower 3,600 rpm motor. A clutch and brake unit 53 is incorporated within the system to allow start up of the machine for high rpm runs and to facilitate the stopping of the rotor in case of emergencies. In addition to employing the vacuum pump 84 to remove the high density atmospheric gas from the drum l2, a supply of helium or other light gas may be coupled to conduit 104, opening up into drum [2 and carrying valve 106 allowing the interior of the drum to be selectively filled with a light gas. Alternatively, this technique may be used without a vacuum system to provide a reduced density gas at atmospheric pressure interiorally of the housing during machining.

The individual work pieces 18 may have selectively applied thereto various masks in the form of coatings or may use masks positioned in contact therewith or spaced intermediate of the work pieces and the discharge openings of the channels 24. The work pieces may comprise steel, glass, silicon and alumina, for instance while such abrasives as 20, $0 and 200 mesh silicon carbide, 20 mesh steel grid and 20 mesh aluminum may fill the feed or supply container 40 for gravity discharge to the center of the wheel. The abrasive particles, in filling the hollow drive shaft 48 at least to the extent of the openings aligned with the narrow inlets 63 of each channel defining spoke 64, permit the particles to pass radially through the hollow spoke and outwardly of the channels 24 under centrifugal force to impact the work pieces 18. By shifting the position of the work piece, impact angles of from 10 to 90 may be achieved. Particle speeds up to 650 feet per second may be easily achieved. Where the work pieces 18 are coated selectively, the mask material may comprise urethane, VINYLSOL, plastic tape, KMER photoresist and MYLAR.

In operation, the housing defined principally by drum I2 is evacuated by operating vacuum pump 84 and the motor 52 is switched on to drive the rotor 10. Abrasive particles 26 are admitted through the feed tube 60 to the hollow shafi 48 and the particles are slung from the center of the rotor out to the radial channels 24 and allowed to strike the work pieces l8 which are mounted around the motor and at a suitable distance from the exit end of the channels 24.

The abrasive leaves the exit end of each radial channel 24 at an angle of about 45 to the tangent on the perimeter of the wheel at that point. The distance of the work pieces 18 from the wheel determines how divergent the steam of abrasive is and also the percentage of total abrasive flow which strikes a given size work piece. If the work pieces are positioned close to the wheel, the stream is more divergent and cuts a more beveled edge on the work. A more vertical edge may be achieved by increasing this distance if a lower abrasive flux (pounds of abrasive per minute striking one square inch of work area) can be tolerated. Obviously, it may be desirable to make this distance fairly large if a large enough number of parts are to be run to take advantage of all the abrasive striking over the increased work area. The abrasive flux due to a lack of air currents and other effect because of the negligible density of the atmosphere is uniform around the perimeter of the wheel so that the cutting effect is independent of where the work is mounted. Of course, this may vary somewhat from the top of the wheel channel to the bottom and to compensate for this the work pieces and in particular the annular supports 16 maybe raised or lowered by adjusting the mounting pins during operation to adjust for the same. Due to the presence of vacuum, it may be necessary to plug the outlet pipes 38 during operation and collect the abrasive at the bottom of the tank, however, it is envisoned that collecting and recirculating means may be employed without disturbing the atmospheric content within the housing. Further, since there may be some destruction to the work pieces during machining, means for filtering and grading of the abrasive is desirable.

Referring to FIG. 5, the material removing range for glass is illustrated graphically. Further the effect of varying the impact angle may be seen in conjunction with various grit sizes at various velocities. For instance by employing 50 mesh particles traveling at an average velocity of 610 feet per second the removal rate increases appreciably. By varying the impact angle from 30 to with a rate increase of approximately 200 percent. The effect of varying mesh size of the abrasive material and velocity are readily apparent from the curves. The removal rate approximately doubles by doubling the velocity from 380 to 610 feet per second while a similar effect is achieved by increasing the size of the particles. The vertical removal rates shown on the right hand coordinate of the graph is calculated by assuming an abrasive flow rate of 20 pounds per minute and a work surface of I00 inches square. Predicted removal rates are also illustrated in the graph and a comparison of the prediced values with the actual removal rates may be readily seen.

Edge definition and cleanness may be affected by a portion of the abrasive bouncing off the edge of the mask and failing to strike the exposed work surface next to the edge. The employment of a finer abrasive tends to improve the edge since less of the abrasive strikes the edge of the mask. Several materials may be employed to mask the work and restrict material removal. Preferably liquid urethane for glass blanks is used in which the blanks are coated with the liquid material and cured. The desired pattern is first cut in the coating and the urethane stripped away from the portions to be machined. This material adheres to the blanks defining the work pieces and has excellent resistance to abrasive action. It is also envisioned that silk screening may be employed for applying the mask material while highly abrasive resistance photoresists have excellent use in machining of parts requiring close tolerances.

In addition to controlled material removal from machining, the free particle impact process of the present invention is use ful for shot peening, particle impregnation, and finishing,

especially of metal work pieces. Peening tends to stress relieve parts and to selectively work harden surfaces, Further, high velocity particles of tungsten carbide or other hard materials can be embedded in softer metal surfaces to improve surface hardness. The free particle impact process of the present invention is thus adapted to a batch type operation in which the parts are arranged about the full perimeter of the rotor. in this manner, all the abrasive being run through the machine may be utilized rather than striking the interior walls of the housing 12.

From the above, it is easily seen that the free particle impact process in a low density environments has excellent capabilities for certain special machining applications. In particular, it is adapted to the machining of ceramics and other hard metal materials The material removal rates obtainable with these materials are relatively high, up to 3.6 inches per minute, vertical cuts with glass for instance and may be attained with good dimensional control. Material removal rates of a ductile material such as steel are higher than those conventionally obtained by abrasive blasting. Further, the process of the present invention is not necessarily confined to a blanking type operation nor machining or cutting flat stock. Cuts can be made partially through and at various angles through the work surface to obtain fairly complex shapes.

What is claimed is:

l. A process for machining workpieces comprising: impacting said work pieces with freely moving high speed abrasive particles in a low density gaseous environment, said workpieces and said particles being in said low density gaseous environment throughout the machining operation, and said par ticles being imparted a high speed movement by centrifuging said particles.

2. The machining process as claimed in claim 1 wherein:

said low density gaseous environment is at least a partial vacuum.

3. The machining process as claimed in claim 2, wherein:

said low density gaseous environment consists primarily of a gas whose density is considerably less than air at atmospheric pressure.

4. The process as claimed in claim 3 wherein said gas is heli 5. The machining process as claimed in claim 4 further comprising the step of varying the impact angle between said work piece and said high speed freely moving abrasive particles.

6. The machining process as claimed in claim 2 wherein said step of moving said abrasive particles in a low density gaseous environment comprises moving the same under vacuum con ditions.

7. The machining process as claimed in claim 1, wherein said step of moving said abrasive particles through a low density gaseous environment comprises enclosing said particles and said work piece in a gas at atmospheric pressure whose density is considerably less than air.

8. An improved apparatus for machining work pieces comprising: a housing defining a closed chamber, means for rotatably supporting a high speed rotating wheel within said housing for rotation about its axis, means for delivering abrasive particles to the center of said rotating wheel, at least one radial channel carried by said wheel and adapted to receive said abrasive particles, means for supporting a plurality of work pieces in circumferential fashion about said rotating wheel and spaced radially therefrom, and means for maintaining a low density gaseous environment within said chamber whereby; said work pieces are simultaneously machined by free particle impact due to slinging of the abrasive particles under centrifugal force from the wheel channel and against said work pieces with reduced windage effects on said wheel, low drag on the abrasive particles and the elimination of cushioning due to the entrapment of high density gas between the particles and the workpiece. I

9. The apparatus as claimed in claim 8 wherein said means for maintaining in said chamber a low density gaseous environment comprises a filter carried internally of said housing, and a vacuum pump coupled thereto.

10. The apparatus as claimed in claim 9 further comprising means for allowing controlled delivery of a gas which is lighter than air at atmospheric pressure to said chamber.

11. The apparatus as claimed in claim 9 wherein said rotating wheel included a hollow, central hub, a plurality of hollow spokes extending radially outwardly from said hub and forming diverging channels spaced circumferentially about said wheel and said apparatus further includes an abrasive particle supply drum closed to the outside atmosphere and stationarily mounted above said rotating wheel, and means for delivering abrasive material to said hub for centrifugal discharge from the outer ends of said channels carried by said spokes.

12. The apparatus as claimed in claim ll wherein the interior wall of said housing and the interior wall off said hollow spokes are coated with an abrasive resistant material.

13. The apparatus as claimed in claim 8 wherein:

a shaft is coupled to said wheel and said apparatus further includes bearings for rotatably supporting the same, said bearing being within said low density gaseous environment within said housing, an oil supply, an oil sump, oil passage means leading from said oil supply to said bearings and from said hearings to said sump, a vacuum pump coupled to said oil sump for subjecting the same to vacuum pressure, and a regulator for maintaining a lesser vacuum within said oil sump than outside of said bearing, thereby placing said bearing under a relatively greater pressure than its surroundings, but below the pressure of said oil supply to cause oil flow from said oil supply, to said bearing and thence to said sump.

1 i l l I 

1. A process for machining workpieces coMprising: impacting said work pieces with freely moving high speed abrasive particles in a low density gaseous environment, said workpieces and said particles being in said low density gaseous environment throughout the machining operation, and said particles being imparted a high speed movement by centrifuging said particles.
 2. The machining process as claimed in claim 1 wherein: said low density gaseous environment is at least a partial vacuum.
 3. The machining process as claimed in claim 2, wherein: said low density gaseous environment consists primarily of a gas whose density is considerably less than air at atmospheric pressure.
 4. The process as claimed in claim 3 wherein said gas is helium.
 5. The machining process as claimed in claim 4 further comprising the step of varying the impact angle between said work piece and said high speed freely moving abrasive particles.
 6. The machining process as claimed in claim 2 wherein said step of moving said abrasive particles in a low density gaseous environment comprises moving the same under vacuum conditions.
 7. The machining process as claimed in claim 1, wherein said step of moving said abrasive particles through a low density gaseous environment comprises enclosing said particles and said work piece in a gas at atmospheric pressure whose density is considerably less than air.
 8. An improved apparatus for machining work pieces comprising: a housing defining a closed chamber, means for rotatably supporting a high speed rotating wheel within said housing for rotation about its axis, means for delivering abrasive particles to the center of said rotating wheel, at least one radial channel carried by said wheel and adapted to receive said abrasive particles, means for supporting a plurality of work pieces in circumferential fashion about said rotating wheel and spaced radially therefrom, and means for maintaining a low density gaseous environment within said chamber whereby; said work pieces are simultaneously machined by free particle impact due to slinging of the abrasive particles under centrifugal force from the wheel channel and against said work pieces with reduced windage effects on said wheel, low drag on the abrasive particles and the elimination of cushioning due to the entrapment of high density gas between the particles and the work piece.
 9. The apparatus as claimed in claim 8 wherein said means for maintaining in said chamber a low density gaseous environment comprises a filter carried internally of said housing, and a vacuum pump coupled thereto.
 10. The apparatus as claimed in claim 9 further comprising means for allowing controlled delivery of a gas which is lighter than air at atmospheric pressure to said chamber.
 11. The apparatus as claimed in claim 9 wherein said rotating wheel included a hollow, central hub, a plurality of hollow spokes extending radially outwardly from said hub and forming diverging channels spaced circumferentially about said wheel and said apparatus further includes an abrasive particle supply drum closed to the outside atmosphere and stationarily mounted above said rotating wheel, and means for delivering abrasive material to said hub for centrifugal discharge from the outer ends of said channels carried by said spokes.
 12. The apparatus as claimed in claim 11 wherein the interior wall of said housing and the interior wall of said hollow spokes are coated with an abrasive resistant material.
 13. The apparatus as claimed in claim 8 wherein: a shaft is coupled to said wheel and said apparatus further includes bearings for rotatably supporting the same, said bearing being within said low density gaseous environment within said housing, an oil supply, an oil sump, oil passage means leading from said oil supply to said bearings and from said bearings to said sump, a vacuum pump coupled to said oil sump for subjecting the same to vacuum pressure, and a regulator for maintaining a lesser vacuum within said oil sump than outside of said bearing, therEby placing said bearing under a relatively greater pressure than its surroundings, but below the pressure of said oil supply to cause oil flow from said oil supply, to said bearing and thence to said sump. 